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Introduction <br />The PECOCO Impoundment ( PECOCO) is located in the NW /4 NW /4 Section 2, <br />Township 5 North, Range 87 West (see Exhibit 13 -2A, Postmining Topography <br />and Permanent Features) in a tributary to Little Grassy Creek. The <br />contributing watershed is approximately 48.1 acres, of which 87 percent is <br />reclaimed and the other 13 percent undisturbed. The impoundment was <br />created as a result of the final mine excavations of the west Lennox Pit in <br />1979. <br />The embankment, a homogeneous earthen fill structure, was constructed in <br />1987 to control the impoundment water level and discharge. The top width <br />of the embankment is 15 feet and the upstream and downstream slopes are <br />4.5H:1V and 3H:1V, respectively. The principal spillway is a 24 -inch <br />corrugated metal barrel pipe. The emergency spillway is a trapezoidal open <br />channel with a bottom width of 10 feet and side slopes of 5H:1V and <br />2.5H:1V, for the right and left slopes respectively. <br />1990 Design Analysis <br />PECOCO was originally designed and constructed to meet or exceed the design <br />and performance requirements of the Colorado Division of Minerals and <br />Geology (CDMG) and adhere to the following specifications: <br />1) Provide storage and treatment for the runoff from the 10 -year, 24- <br />hour storm event to assure compliance with receiving water quality <br />standards in accordance with Rule 4.05.6(3)(a). <br />2) Provide storage for the 10 -year, 24 -hour storm event without <br />discharging through the emergency spillway in accordance with Rule <br />4.05.9(2)(b). <br />3) Provide an emergency spillway capable of passing runoff from the 25- <br />year, 24 -hour storm event in accordance with Rule 4. 05. 9 (2) (c) (ii) . <br />Seneca Coal Company (SCC) evaluated <br />the impoundment design in <br />1990 as part <br />of Technical Revision No. 20, SCC's <br />responses to CDMG's mid -term review <br />dated January 17, 1990; the design <br />evaluation is presented in Appendix 7- <br />6g. The SEDCAD+ hydrology and sedimentology computer model <br />was used to <br />evaluate precipitation runoff from <br />the 10 -year, 24 -hour and <br />100 -year, 24- <br />TR-37 <br />1 <br />Revised 10/05 <br />